1. Field of the Invention
The present invention relates generally to tube plugs and more particularly to a retainer assembly for heat exchanger tube plugs.
2. General Background
In tube-type heat exchangers a primary coolant fluid flows through the tubes of the heat exchanger while a secondary coolant fluid flows around the outside and in contact with the tubes such that heat exchange takes place between the two fluids. A defective or corroded tube presents the possibility of a leak wherein the higher pressure primary coolant will exit the tube and mix with the secondary coolant. This results in reduced efficiency of the heat exchanger and also in radioactive contamination of the secondary coolant in a nuclear steam generator. The most cost effective solution to such a problem is to plug the heat exchange tube.
Several types of explosive and mechanical plugs are used in the industry. However, the present invention is aimed mainly at mechanical tube plugs such as that described in U.S. Pat. No. 4,390,042. Some of these types of plugs have been found to be susceptible to Primary Water Stress Corrosion Cracking (PWSCC). This can be a result of the Inconel 600 material either not having a high enough temperature during final mill anneal and then not responding to thermal treatment or the material not receiving the proper thermal treatment. This results in susceptibility to PWSCC at each end of the expanded region where the mandrel has been pulled into the tapered section of the plug to expand the ribs into the tube wall, regions of high tensile stress. The resulting crack propagation may result in loss of the plug nose with primary-secondary leakage, assuming the tube has a through-wall defect, or in loss of the plug head which could become a loose part in the reactor coolant system. There is also the potential of a plug part ejecting up into the U-bend of the steam generator tubing. The solution to prevent such plugs from causing damage to the reactor coolant system is removal of the plugs or installing a retainer in the plug which serves the dual purpose of retaining the cracked plug in position and preventing substantial leakage of primary coolant through the defective plug into the defective heat exchange tube (plugging the plug). Due to the extra time and equipment required for plug removal and the fact that some attempts at plug removal have resulted in separation of the plug threaded/open end from the plug body, installation of a retainer assembly into the existing plug appears to be the most efficient initial solution to the problem. Applicant is not aware of any prior art aimed at solution of the problem associated with this particular plug design since the problem has only recently been recognized.